This invention relates to an electron source for emitting a flow of electrons, particularly but not exclusively a fast response electron source for cathode ray tubes, image pick-up devices, display devices or electron lithography. The invention further relates to equipment having such electron sources.
U.K. Patent Specification (GB-A) No. 830,086 discloses an electron source comprising a semiconductor body, and an n-p-n structure formed in the body by a p-type first region between n-type second and third regions. Electrons are generated in said n-p-n structure for emission into free space from a surface area of said body after flowing from the second region through the first and third regions. An advantage of this n-p-n structure (a specific example of which is illustrated in FIG. 3 of GB-A No. 830,086) is that the electron source can operate with voltage levels below those necessary to cause avalanche breakdown of the semiconductor. Examples of other electron sources which have a simple p-n structure but which are operated in avalanche breakdown are also described in GB-A No. 830,086.
Each region of the n-p-n structure disclosed in GB-A No. 830,086 has an electrode connected to a voltage supply for operating the structure in a manner similar to a transistor. The first p-n junction which is between the second and first regions is biased in the forward direction like an emitter junction. The second p-n junction between the p-type first region and the n-type third region is biased in the reverse direction like a collector junction. Only a small saturation current flows across the second p-n junction in the absence of any injection of electrons from the first p-n junction. The electrons injected into the p-region diffuse across the p-region and are accelerated to high energies by the potential drop across the second p-n junction. By having a very thin n-type third region coated with a material reducing the electron work function, some of these electrons escape into free space before losing their energy to the lattice. The amount of such electron emission into free space is adjusted by varying the voltage of the voltage supply applied across the first p-n junction between the second and first regions.
However such an n-p-n electron source as disclosed in GB-A No. 830,086 has several disadvantages. The electrons injected into the p-type region and the holes injected into the n-type second region constitute minority charge-carriers which lead to charge-storage time delays in the switching rate of the device, similar to those occurring with n-p-n bipolar transistors. This limits the rate at which the electron source can be switched to vary the electron flux emitted by the device.
In practice only a small proportion of the accelerated electrons emerges from the surface area (in spite of the coating on the thin third region). The much larger proportion of electrons which are not emitted are extracted from the device as a current flow from the electrode connection of the third region. It is desirable to have a very thin third region in order to maximize the number of electrons emerging from the surface area. A thickness range of 0.01 to 10 micrometers is mentioned in GB No. 830,086. However in order to act as a n-p-n transistor structure with base control of the collector current, the n-type third region of the device described in GB No. 830,086 cannot be very highly doped compared with the first and second regions without degradation of the transistor emitter efficiency. Therefore, in practice if its thickness is significantly less than about 1 micrometer, the third region will have a high electrical resistance. Thus, the rate at which the electron source can be switched will be further limited by the R.C. time constant resulting from this high collector resistance and associated junction capacitance. Furthermore because the n-type second region needs to be highly doped for good transistor emitter efficiency its p-n junction with the p-type first region will have a large capacitance which must be charged via the base resistance of the transistor structure so further limiting the response rate of the electron source.
The electrode connections to each region of the n-p-n structure are indispensable to the operation of the device disclosed in GB No. 830,086. This requirement for three separate electrode connections complicates the structure of the electron source and its manufacture in a reliable manner, particularly if it is desired to fabricate a two-dimensional array of such devices in a common semiconductor body. Such two-dimensional arrays are desirable for image pick-up devices, display devices and electron lithography. Furthermore in order to provide the intermediate p-type region with a sufficient contact area for its electrode connection, it is generally necessary to extend the p-type region over a surface area alongside the n-type third region, but this increases the p-n junction area and associated capacitance and therefore tends further to reduce the response speed of the electron source.